Reluctance-type angle sensors, such as resolvers and synchros, used for determining angular position and/or angular speed of a rotor or rotating shaft are well known in the art. These types of sensors may adopt different forms and designs depending on the application.
A commonly used type includes a stator and a rotor that is rotatable about a concentric axis with respect to the stator and separated from it by an air gap. The stator includes exciting coils or magnetic poles for generating a magnetic flux distribution that extends over the air gap to the rotor, and whose intensity in the air gap is changed when the rotor rotates with respect to the excited stator. The changes in the magnetic flux induce a measurable phase controlled electromagnetic force, which can be detected, for instance, by sensing windings or coils. The angular position of the rotor (with rotating shaft) with respect to the stator may be obtained by analyzing the phase of the detected signals.
Under this type of angle sensors are included the so-called passive reluctance resolvers in which both the exciting and sensing coils are wound on the stator according to a pre-determined winding scheme. The change of magnetic flux is achieved by using a rotor of a soft magnetic material with a non-regular external shape, for instance, provided with lobes or indentations on the surface facing the stator. When the rotor rotates, a variation of the air gap and consequently a change of magnetic reluctance between the rotor and the opposed stator is produced. The particular winding scheme allows the exciting and sensing coils to be influenced differently and therefore, to derive the angular position of the rotor from the electromotive voltage induced by this change.
FIG. 1 shows an exploded view of a conventional synchro-resolver.
The synchro-resolver 100 includes a flange 110 adapted to be mounted onto a mounting structure, such as a frame (not shown). A rotor 115, formed by an assembly of a rotor body 120 and a shaft 125 to which the rotor body 125 is rigidly attached to, is held in position with a spacer ring 130, a circlip 135, a bearing 140 and an E clip 150, which are provided at one side of the rotor shaft 125, and a second bearing 140 that is provided on the opposite side. The rotor shaft 125 is to be engaged with a prime mover (not shown). During operation, the rotor shaft 125, also called follower, spins about its rotation axis Z along with the prime mover. An inner cover 160 seals the rotor 115 and provides a protective covering against any spillage/penetration of any invasive fluids even with higher pressures, thereby serving as a sealing means.
The synchro-resolver 100 further includes a stator 170, which is conventionally formed by a stacked-up lamination containing a number “N” of poles 175 (in the illustrated example, N=12) depending upon the magnetic force needed to be generated on the rotor.
An outer cover 180 provides hermetical sealing against the impact of all external environmental factors that might affect the performance of the reluctance type resolver 100.
The stator poles 175 are provided at regular spaces along the inner perimeter of the stator, and projecting radially inwards towards the rotor axis Z. Exciting coils or windings for generating the magnetic field and sensing coils for detecting the electromagnetic force induced by the rotation of the rotor are wound on the poles 175 (not shown). The winding on each pole is suitably done according to a pre-determined winding scheme.
The number and winding scheme depends on the specific application, such as desired resolution of the synchro-resolver, range of angular speed to be detected, etc., and is done in such a way that the output electromotive force is resolved into the respective sine and cosine waves corresponding to the angular position of the rotor shaft. Examples of specific winding schemes for detecting the angular position of the rotor are fully described in document WO 2010/130550 and references cited therein, and therefore, shall not be described in full detail here.
Document WO 2010/043586 A2 describes another example of a passive reluctance resolver in which the variation of magnetic flux is produced by the distribution of the magnetic mass in a regularly shaped rotor. The so-called half-magnetic rotor has a body with a half made of a soft magnetic material, the magnetic half, and another half made of a non-magnetic material, the balancing half. The magnetic and balancing halves are designed such as to be joined together at a plane that is inclined with relation to the rotational axis of the rotor. The distribution of magnetic mass is such that a sinusoidal change of magnetic flux can be produced when the rotor is rotated.
For both rotor variants, the excited stator generates a magnetic field that extends to the rotor over the air gap and which is distributed over the rotor. Thus, the magnetic force acting on the rotor is one of the decisive factors for the accuracy of the angular measurement of reluctance-type angle sensors.
However, when the stator generates an excitation of magnetic field with which the rotor interacts, there is an appreciable amount of magnetic flux that is not utilized due to the residual gaps and interim pole gaps present between them. In particular, an amount of the produced magnetic flux is lost as divergent fields around the rotor ends, pole gaps, etc, which result in a loss of effective magnetic force acting on the rotor due to the divergence of magnetic flux lines. Further, there might be a formation of eddy currents around each pole of the stator due to self-induction.
The magnitude of the produced magnetic flux that actuates on the rotor is then limited as there is no system to compensate for the loss of magnetic flux due to divergent fields on the rotor surface and spillage across the corners and the pole gaps.
Although a higher current in the stator coil may be used for producing a higher intensity of magnetic flux, this has the disadvantage that more energy is consumed for generating the required output. Further, the improvement reached through an increase of ampere-turns in the stator coil could be only marginal if no other mechanism is used for counterbalancing the lost of divergent magnetic flux.
These factors lead to a loss of the effective magnetic force acting on the rotor. Hence, there is a need for a technique capable of reducing the loss of effective magnetic force acting on the rotor due to the formation of divergent magnetic fields, eddy currents, etc, around each pole of the stator and the rotor surface.
This has poised to think of a technique for effectively using the generated magnetic flux so as to minimize the energy required for excitation of the stator as well as to increase the resolution of the magnetic field vectors resulting from the interaction of the rotor field with that of the stator.